Preheat oven to 375°.
Place a medium nonstick skillet coated with cooking spray over medium-high heat. Add the chopped onion, and sauté for 3 minutes. Combine the onion, 1/4 cup ketchup, 1 tablespoon mustard, mozzarella, and next 6 ingredients (mozzarella through egg) in a large bowl. Crumble ground meats over cheese mixture; stir just until blended.
Pack mixture into an 8 x 4-inch loaf pan coated with cooking spray. Combine 2 tablespoons ketchup and 1 tablespoon mustard; spread over top of loaf. Bake at 375° for 1 hour or until meat thermometer registers 160°. Let meat loaf stand in pan 10 minutes.
Remove meat loaf from pan, and cut into 12 slices.
Note: Substitute lean ground beef for the ground veal and pork, if desired.